Liquid storage tanks frequently contain floating roofs that float on top of the liquid contained in the tank. Such roofs are vertically displaceable with the addition of liquid to, or removal of liquid from, the tank. Certain industries, such as those associated with petrochemical processing or storage, including refineries, tank terminals and the like, often use floating roofs to minimize product economic losses and for environmental reasons to reduce the emission of volatile hydrocarbons. Floating roofs also protect the product contained within the tank from debris, excess water addition, or other contaminants. For those storage applications involving flammable liquids, floating roofs are used to reduce the risk of fire and/or explosion by reducing the exposed vapor space in the tank above the flammable liquid. Floating roof rim seals and shunts are also frequently used along with a floating roof to minimize the risk of rim fires.
The risk of a severe roof failure accident can be associated with several potential causes, including the lack of adequate overfill warning or notification of an alarm condition. The floating roof may also get stuck at one or more rim locations during filling or emptying of the tank due to icing, the accumulation of debris, or an obstruction at the rim seal. Improperly monitored floating roofs may also experience leaks, excessive snow or water load on the top of the roof, freezing of accumulated or pooled water on top of the roof, or excessive wind pressure. These and other such conditions may lead to failure of the roof or improper position and/or inclination (tilt) of the roof. Safe operation of the storage tank therefore requires a reliable and accurate way to monitor the status, particularly the position and inclination, of a floating roof.
An additional concern for the storage of flammable and/or explosive liquids within the tank, and the monitoring of the floating roof, relates to the use certification for any monitoring or measurement devices used on or near the tank. Importantly, such devices must generally meet the National Fire Protection Association (NFPA) Class 1, Division 1 or more stringent certification requirements where, along with other conditions, ignitable concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors can exist under normal operating conditions.
Due to the importance of monitoring floating roof operation, including providing accurate and current position and inclination information for floating roofs, and the absence of cost-effective and reliable means for monitoring such roofs, a continuing need exists for improved systems and methods for the cost-effective monitoring of floating roofs in liquid storage tanks.